The invention concerns an optical unit for fiber optic cables, generally used in telecommunications, said unit comprising a plurality of optical fibers grouped in a tube and possibly assembled into one or more ribbons inside the tube. It is more particularly concerned with the material of the tube of a unit of this kind containing the optical fibers.
A cable comprising a unit of the above kind is used in telecommunications and in particular in distribution systems with high levels of branching where a plurality of such cables are assembled to form a macro-cable at the head end with a high density of optical fibers.
FIGS. 1 and 2 show in cross-section two prior art cables with a Unitube.RTM. structure (FIG. 1) and a stranded loose tube structure (FIG. 2), respectively. A prior art stranded loose tube structure cable is described in document FR-A-2 280 911, for example.
The optical fiber cables 10 and 20 comprise optical fibers 2 in the form of bundles or ribbons loosely received in a tube 3 the longitudinal axis of which is that of the cable (Unitube.RTM. structure) or in a plurality of tubes 3 wound around a central strength member 1 made of a dielectric material (stranded loose tube structure). An optical unit 5 comprises a tube 3 and the optical fiber(s) 2 it contains.
Around the unit 5 of the Unitube.RTM. structure cable 10 or the set of units 5 of the stranded loose tube structure cable 20 are disposed in succession a mechanical reinforcing layer 6 consisting of dielectric reinforcing members, for example, and a protective outer sheath 7 made from an insulative material. The empty spaces between units 5 of the cable 20 can be filled with a filler material, for example a filler gel 4, providing a longitudinal seal, or contain elements that swell in the presence of a liquid to provide the seal.
The tube(s) 3 of the units 5 containing the optical fibers 2 are conventionally made of a hard rigid material, such as high-density polyethylene (HDPE) or polybutylene terephtalate (PBT); they can equally comprise a double polybutylene terephtalate/polycarbonate (PBT/PC) layer. According to document FR-2 280 911 the tubes of the stranded loose tube structure can also be of polyethylene or polypropylene.
The Young's modulus of the material of the tubes 3 in the prior art cables 10 or 20 is generally high, in the order of 2 400 MPa to 2 600 MPa at 20.degree. C. Tubes of this type provide good mechanical protection for the optical fibers 2, especially against radial compression loads. Tubes made from a material with a lower Young's modulus must be relatively thick to provide good resistance to radial compression of the cable. In this case the thickness of the tubes 3 is relatively great (typically in the order of 0.5 mm for stranded loose tube structures and 0.75 mm for Unitube.RTM. structures). Accordingly the cables 10 or 20 obtained are very bulky and/or very stiff, and consequently difficult to handle.
For this reason European patent application EP-A-0.769.711 proposes to use flexible and elastic materials for the tube of the fiber optic unit(s); in particular it proposes an optical unit for a fiber optic telecommunication cable comprising a tube made from a plastics material in which at least one optical fiber is loosely received, characterized in that the thickness of said tube is less than or equal to 0.5 mm and in that said material has a Young's modulus less than 1 500 MPa at 20.degree. C. and a stress/strain curve with no yield point. A material of the above kind makes cables that are less bulky and easier to handle than the prior art cables previously referred to.
Confinement of the optical fibers in the tube also raises the problem of increased attenuation caused by microcurvature. The tube must be able to deform under external mechanical or thermal load in a way that does not generate microcurvature of the optical fibers tightly received inside the tube.
Connecting a cable including an optical unit of this kind also raises the problem of cutting the tube. Cutting must be easy and must not entail any risk of damaging the optical fibers tightly received inside the tube.